Historically, access to computing resources and data may be limited to authorized users. Typically, an authorized user may be authenticated by a username-password pair. The password is kept confidential, but storage of passwords on a host presents a risk of compromise, for example, by a hacker. Two-factor authentication improves security, but still requires a password and adds additional burden on the user. Passwords are increasingly problematic because complex passwords are hard to remember for humans; people reuse passwords at several sites, thus increasing the risks; and forced rotation of passwords for compliance reasons often ends up weakening security of the system. The need remains for improvements in user authentication.